Method of treating boiler make up water

ABSTRACT

Hot condensate from feed water heaters is drawn off and mixed with make-up water, and the mixture heated and deaerated using turbine exhaust steam.

United States Patent [191 Tidball et al.

[11] 3,820,333 June 28, 1974 METHOD OF TREATING BOILER MAKE-UP WATER [75] Inventors: Robert A. Tidball, Swarthmore, Pa.; Gene F. Lauro, Greenfield Heights,

[73] Assignee: Foster Wheeler Corporation,

Livingston, NJ.

[22] Filed: Feb. 24, 1971 [2]] Appl. No.: 118,255

[52] US. Cl. 60/667 [51] Int. Cl. F0lk 7/40, F0lk 7/38 [58] Field of Search 60/67, 94, 95, 106, 107,

[56] References Cited UNITED STATES PATENTS 1,723,257 8/1929 Sim 60/94 R 1,749,707 3/1930 Lang 150/94 R 2,781,640 2/1957 Riehl 60/95 R 3,003,321 10/1961 Warth 150/95 R X 3,016,712 1/1962 Taylor 60/107 3,135,096 6/1964 Schroedter 60/107 X 3,194,020 7/1965 Hanzalek 60/95 R X 3,238,729 3/1966 Frankel et al 60/107 X 3,411,299 11/1969 Nettel 60/95 R X Primary ExaminerMartin P. Schwadron Assistant Examiner- -Allen M. Ostrager FirmSeidel, Gonda &

Attorney, Agent, or Goldhammer [5 7] ABSTRACT Hot condensate from feed water heaters is drawn off and mixed with make-up water, and the mixture heated and deaerated using turbine exhaust steam.

14 Claims, 2 Drawing Figures METHOD OF TREATING BOILER MAKEUP WATER This invention relates to a method of treating boiler make-up water, and more particularly, to a method of providing preheated deaerated make-up water into a thermal power cycle when the quantity of make-up is relatively large compared with the total steam flow. Still more particularly, this invention relates to a method wherein waste heat from the drains of feed water heaters is used to preheat boiler make-up water prior to its entry into the thermal system.

In modern thermal electric power stations, the amount of make-up water needed in the thermal cycle is quite small, generally on the order of a few percent of the total steam flow. In normal operation, it is necessary only to make up for scheduled blowdown operations in the boiler, and to replace water lost through various packing glands and valves, and in the turbine where some steam escapes to atmosphere. Make-up water is initially introduced into the system through either an evaporator or a demineralizer. The system includes a make-up storage tank from which the systems make-up requirements are drawn. During operation, the make-up may be introduced through a spray system into a turbine exhaust condenser, there to be deaerated and heated to saturation.

In a typical steam cycle, steam generated in a boiler is fed to a turbine, where it is expanded to generate power. A major portion of the steam passes completely through the turbine, and exhausted into a condenser. Exhaust steam in the condenser gives up heat by indirect heat exchange to a circulating coolant, and the resulting condensate is gathered in a hotwell. The coolant is drawn from any convenient source, such as a river, lake or sea, and conventionally, is returned to its source after passing through the condenser tube bundle.

The condensate is then pumped through feed water heaters to the boiler.

An occasional programmed blowdown from the boiler is necessary to keep dissolved solids of the boiler water within specified limits. Blowdown, and the above-mentioned operating losses, require that makeup water be added to the system to maintain a constant inventory. Also, in some instances it is economically desirable to withdraw large quantities of steam from the boiler before the steam reaches the turbine, or to withdraw large quantities of steam at some extraction point within the turbine for use as process water in a chemical process or other such non-power producing operation. In some instances, the quantity of steam thus withdrawn is a large percentage (sometimes above 50%) of the total steam generated. In the foregoing instances, large quantities of make-up water may be needed.

For scale and corrosion control it is necessary to re move dissolved non-condensible gases from the makeup water prior to introduction of the water into the thermal cycle. Dissolved gases from the air, the most significant of which are oxygen and carbon dioxide, are present in all naturally occurring waters. The amount of dissolved gases found in water increases as the water temperature decreases or as the pressure over the water is increased. Therefore, in adding make-up water, it is desirable to heat the make-up to as near its saturation temperature as possible at the point of input.

When this is done, a higher degree of deaeration is achieved.

In the known technique of adding make-up as a spray in the turbine exhaust condenser, heating of the makeup water is achieved as follows: The water is sprayed in droplet form into the steam lanes around the condenser tubes. Exhaust steam sweeping past the falling droplets heats the droplets to near saturation temperature, and carries off non-consensible gases given up by the droplets during heating. Heating and deaeration do not occur instantaneously. Thus, the make-up water must exist in droplet form for a time sufficient to allow the droplets to be heated to near saturation temperature, and for the dissolved gases to diffuse to the droplet surfaces. With small quantities of make-up at temperatures not too far below saturation temperature in the condenser, suitable heating and degassing occur before the droplets fall into the hotwell. However, where large quantities of make-up water are involved, or during cold water operation in the winter season (when makeup water may be as low as 32 F. heating to saturation or near-saturation temperature does not occur in the condenser, nor does full deaeration.

In the method in accordance with this invention, large quantities of makeup water are partly or completely preheated prior to entering the condenser, and without the use of external energy sources. Thus, with the present invention, the addition of large quantities of make-up water to the cycle becomes practical. In the present process, high-temperature drains or condensate from feed water heaters is mixed with incoming make-up water prior to introduction of the make-up water into the turbine exhaust condenser. The resulting mixture thus enters the condenser with a temperature nearer the saturation temperature than the make-up water would otherwise have.

For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a flow chart of a steam cycle using techniques in accordance with the present process.

FIG. 2 is a cross-sectional view of apparatus used in practicing the present process.

Referring now to the drawings in detail, wherein like numerals indicate like elements, there is seen in FIG. 1 the flow diagram of a typical power cycle incorporating the present process. In the illustrated cycle, steam generated in a boiler 10 is fed through a line 12 to a turbine 14, where it is expanded to generate power.

Most of the steam passes completely through the turbine 14, and is exhausted through an appropriate duct 16 into a condenser, designated generally by the reference numeral 18. In the condenser 18, the exhaust steam encounters condenser tubes 20, where-upon it condenses to water, and drops to a hotwell 22. Heat rejected by the condensing steam is given up to coolant water circulating in the condenser tubes 20. Thus, water from any convenient source, such as a river, lake or the sea, enters an inlet line 24, passes through the condenser tubes 20, and is discharged to its source through a line 26.

From the hotwell 22, the condensate passes through a line 28, to a condensate pump 30, and from thecondensate pump 30 through an ejector condenser 32 and through low pressure feed water heaters 34 and 36 to a deaerating feed water heater 38.

Auxiliary steam, tapped from the boiler or another source enters the ejector condenser 32 by way of a line 40, and passes from the ejector condenser 32 to the condenser 18 through a line 42.

From the deaerating feed water heater 38 the condensate, which will hereafter be referred to as feed water, passes through a line 44 to a booster pump 46, and is pumped by the booster pump 46 through high pressure feed water heaters 48 and 50. A boiler feed pump 52 pumps the heated water through a line 54 to the boiler 10.

Heat is supplied to the low pressure feed water heaters 34 and 36, the deaerating feed water heater 38, and the high pressure feed water heaters 48 and 50 by steam from the turbine 14. Thus, steam leaving the turbine 14 at extraction points 54,56, 58, 60 and 62 flows through respective lines 64, 66, 68, 70 and 72 to the various feed water heaters 34, 36, 38, 48 and 50. Cascading drain lines 74 and 76 interconnect the high pressure feed water heaters 48 and 50 and the deaerating feed water heater 38. A drain line 78 interconnects the low pressure feed water heaters 34 and 36 in cascade fashion.

The manner in which make-up water is added to the cycle will now be described.

A make-up storage tank 80 is provided. The storage tank 80 receives make-up water from a demineralizer, not shown, and also receives through a line 82 surplus condensate dumped from the system. Make-up water from the make-up storage tank 80 is brought to the condenser 18 by an inlet line 84. A line 86 carries drains from the low pressure feed water heater 34 to the inlet line 84.

Referring now to FIG. 2, the manner in which the line 86 conveys drains to the inlet line 84 is seen in greater detail.

Drains from the low pressure feed water heater 34, including drains cascaded from the other feed water heaters, emerges from the line 86 at a nozzle 88, directly into a make-up stream in the inlet line 84. The resulting mixture passes through a spray head 90 adjacent tube bundles 20" within the condenser 18.

The temperature of drains taken from even the lowest pressure feed water heater 34 is far above the temperature of the make-up water, and often near or above 212F. Thus, the make-up water is brought nearer to saturation temperature in the condenser to allow better deaeration and increased thermal efficiency.

As an example of the efficacy of the present process, consider a condenser 18 operating at 1 /2 inches mercury backpressure, or about 92 F, with a lowest pressure heater 34 condensing 450,000 lbs./hr. of water at 190 F. Make-up to the condenser 18 is supplied at 1,000,000 lbs./hour and 40 F. A heat balance for this system is expressed in the following equation:

F. Since this is only 6 F below the saturation point in the condenser, the resulting mixture can be sprayed into the lanes where deaeration and reheat to acceptable levels can be expected.

A similar technique can be used in connection with make-up applied at the deaerating feed water heater 38. Referring to FIG. 1, make-up water from an evaporator 92 is introduced to the system at the deaerating feed water heater 38. Heat for the evaporator 92 is provided by a tap 94 off the steam-carrying line 68. Spent steam or condensate from the evaporator joins the drain line 76 from the feed water heater 48. An inlet line 96 conducts the make-up water into the line 76.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. A method of treating boiler make-up water for a steam cycle using a boiler, a turbine receiving steam from the boiler, and feed water heaters providing heated feed water for the boiler, and wherein a quantity of turbine exhaust steam is directed to the feed water heaters to heat feed water by indirect heat exchange, comprising the steps of drawing off drains from at least one feed water heater at a first temperature, mixing the drawn off drains with make-up water at a second temperature lower than the first temperature, deaerating the resulting mixture, and then passing the resulting mixture through the feed water heaters to the boiler.

2. A method in accordance with claim 1 wherein deaerating of the resulting mixture is performed by direct contact of the mixture with turbine exhaust steam.

3. A method in accordance with claim 1 wherein deaerating of the resulting mixture is performed by directing a spray of said resulting mixture into a stream of turbine exhaust steam.

4. A method in accordance with claim 3 wherein the resulting mixture is heated by the turbine exhaust steam during deaeration to substantially its saturation temperature.

5. A method in accordance with claim 1 wherein turbine exhaust steam is directed to a condenser, and the resulting mixture introduced into the condenser to be heated and deaerated therein by the turbine exhaust steam.

6. A method in accordance with claim 5 wherein the resulting mixture is introduced into the condenser as a spray in direct vapor contact with the turbine exhaust.

7. A method in accordance with claim 5 wherein the resulting mixture is heated in the condenser to substantially its saturation temperature therein.

8. A method in accordance with claim 1 wherein the step of drawing off drains from the feed water heaters is performed by cascading drains from successive heaters, the make-up water being mixed with the drains from the lowest pressure feed water heater.

9. A method in accordance with claim 8, wherein the turbine exhaust steam is directed to a condenser, and the resulting mixture being introduced into the condenser and heated and deaerated therein by direct contact with the turbine exhaust steam.

10. A method in accordance with claim 9, wherein the resulting mixture is introduced into the condenser as a spray.

11. A method of introducing make-up water into a steam cycle using a boiler, a turbine receiving steam from the boiler, and feed water heaters providing heated feed water for the boiler by indirect heat exchange, comprising the steps of drawing off drains from the feed water heaters, cascading the drains from successive heaters, mixing the drains with make-up water, deaerating the resulting mixture, and then passing the resulting mixture through the feed water heaters to the boiler.

12. A method in accordance with claim 11, wherein deaerating of the resulting mixture is performed by ditherein. 

1. A method of treating boiler make-up water for a steam cycle using a boiler, a turbine receivIng steam from the boiler, and feed water heaters providing heated feed water for the boiler, and wherein a quantity of turbine exhaust steam is directed to the feed water heaters to heat feed water by indirect heat exchange, comprising the steps of drawing off drains from at least one feed water heater at a first temperature, mixing the drawn off drains with make-up water at a second temperature lower than the first temperature, deaerating the resulting mixture, and then passing the resulting mixture through the feed water heaters to the boiler.
 2. A method in accordance with claim 1 wherein deaerating of the resulting mixture is performed by direct contact of the mixture with turbine exhaust steam.
 3. A method in accordance with claim 1 wherein deaerating of the resulting mixture is performed by directing a spray of said resulting mixture into a stream of turbine exhaust steam.
 4. A method in accordance with claim 3 wherein the resulting mixture is heated by the turbine exhaust steam during deaeration to substantially its saturation temperature.
 5. A method in accordance with claim 1 wherein turbine exhaust steam is directed to a condenser, and the resulting mixture introduced into the condenser to be heated and deaerated therein by the turbine exhaust steam.
 6. A method in accordance with claim 5 wherein the resulting mixture is introduced into the condenser as a spray in direct vapor contact with the turbine exhaust.
 7. A method in accordance with claim 5 wherein the resulting mixture is heated in the condenser to substantially its saturation temperature therein.
 8. A method in accordance with claim 1 wherein the step of drawing off drains from the feed water heaters is performed by cascading drains from successive heaters, the make-up water being mixed with the drains from the lowest pressure feed water heater.
 9. A method in accordance with claim 8, wherein the turbine exhaust steam is directed to a condenser, and the resulting mixture being introduced into the condenser and heated and deaerated therein by direct contact with the turbine exhaust steam.
 10. A method in accordance with claim 9, wherein the resulting mixture is introduced into the condenser as a spray.
 11. A method of introducing make-up water into a steam cycle using a boiler, a turbine receiving steam from the boiler, and feed water heaters providing heated feed water for the boiler by indirect heat exchange, comprising the steps of drawing off drains from the feed water heaters, cascading the drains from successive heaters, mixing the drains with make-up water, deaerating the resulting mixture, and then passing the resulting mixture through the feed water heaters to the boiler.
 12. A method in accordance with claim 11, wherein deaerating of the resulting mixture is performed by directing a spray of the resulting mixture into a stream of turbine exhaust steam.
 13. A method in accordance with claim 12, wherein the turbine exhaust steam is directed into a condenser, and the spray of resulting mixture being introduced into the condenser to be heated and deaerated therein by the turbine exhaust steam.
 14. A method in accordance with claim 13, wherein the spray of resulting mixture is heated in the condenser to substantially its saturation temperature therein. 